Daylighting systems are used to provide natural light to building spaces, reducing the need for electric lighting. Effective use of daylight has several positive benefits including lower energy bills, lower fossil fuel consumption for electricity generation, and increased work environment satisfaction for occupants. An overview of many existing daylighting systems can be found in Ruck et al., Daylight in Buildings: A Source Book on Daylighting Systems and Components, a report of the International Energy Agency SHC Task 21/ECBCS Annex 29 (July 2000) and incorporated herein by reference in its entirety.
Anidolic Systems
The search for a static daylighting system that redirects light deeply and also prevents direct sunlight from entering the room at a downwards angle led to the science of non-imaging optics and a technology called the compound parabolic concentrator (CPC). The field of non-imaging, or anidolic, optics was initially used in the development of solar collectors (Scartezzini and Courret, “Anidolic Daylighting Systems,” Scartezzini, J.-L. & Courret, G., 2002. Anidolic Daylighting Systems. Solar Energy, 73(2), pp. 123-35.), incorporated herein by reference in its entirety.
The CPC was originally used as a solar concentrator that accepts all light rays from a defined angular extent and concentrates them on a smaller area. The CPC, when used for daylighting applications, uses the same type of reflector profile, but light moves through the profile in the opposite direction. Referring to FIG. 1, based on a figure of a side view of a zenithal anidolic collector from Scartezzini, light enters from all directions through a small inlet aperture and is aligned into a controlled angular range at the outlet. See Scartezzini, Jean-louis, “Anidolic Systems—Non-imaging Transmission of Daylight into Darker Parts of Buildings,” EPFL Solar Energy and Building Physics Laboratory LESO-PB/Web 17 Aug. 2010, incorporated herein by reference in its entirety.
A complete discussion of how the CPC works can be found in Winston, Roland, Juan C. Miñano, Pablo Benitez, and W. T. Welford. Nonimaging Optics. Amsterdam: Elsevier Academic, (2005), pages 50 though 57, incorporated herein by reference in its entirety. There are four parameters that define the CPC's geometry: inlet size, outlet size, length, and maximum output angle. Setting any two of these parameters will determine the other two and completely define the geometry of the CPC.
Existing anidolic systems, such as the zenithal anidolic collector, were found to have several major shortcomings when applied to an office building setting. For example, since the zenithal anidolic collector allows specular descending rays, the system typically needs to be shaded under sunny conditions to protect from glare. In an open-plan office, blinds that are shut to control glare often remain shut for long periods of time. This problem may only be fully overcome by automating the shading system to eliminate the need for adjustments by the occupants. The physical dimensions of the zenithal anidolic collector are also quite large, approximately 1 to 2 m long and 0.5 to 1 m tall. This size reduces the ceiling height near the facade and makes using the space underneath awkward. Integrating the exterior light scoop can also be an architectural challenge.
Louver Systems
Reflective louvers form another relevant group of daylighting systems. Louver systems are often designed to be located between two panes of glass, making the task of integration into the facade much easier than with larger systems such as the zenithal anidolic collector. Conceptually, louver systems generally consist of a vertical array of identically-shaped curved slats, with a profile that redirects daylight onto the ceiling. The Fish system (U.S. Pat. No. 4,699,467, incorporated herein by reference in its entirety, and also illustrated in Ruck, page 4-24), shown in FIG. 2, is one such louver system. Although it is able to redirect all incoming light above horizontal, it does a relatively poor job of collimating the output light. As a result, light from these louvers may not penetrate as deeply as desired.
Compagnon evaluated a reflector system comprised of anidolic profiles, as shown in FIG. 3. See p. 138, FIG. 5.47 of Compagnon, R., Simulations Numeriques de Systemes D'Eclairage Naturel a Penetration Laterale. PhD Thesis. Lausanne: Ecole Polytechnique Federale de Lausanne, 1994, incorporated herein by reference in its entirety. The inner anidolic curves are tilted upwards so that light exiting the reflectors may be directed onto the ceiling, to protect from glare. The idea of using anidolic profiles to compose a louver array is an attractive one. However, in this configuration the design of the outer half of the louver results in the rejection of all light above a projected elevation angle of 60°. Light enters the room at a maximum of 60° above horizontal, which is a lower maximum elevation angle than that of the Fish Louver. However, the output elevation range is still fairly wide and the amount of light traveling deeply into the space may be limited as a result. The lower curve of the CPC profile is truncated in this design, meaning that the louvers will spill light below horizontal, potentially causing glare. Also, although it is more compact than the zenithal anidolic collector, the size of the assembly is still rather bulky at 0.48 m depth.
Referring to FIGS. 4a and 4b, the CPC was incorporated into another louver system developed by Eames and Norton (Eames, P. & Norton, B., 1994. A Window Blind Reflector System for the Deeper Penetration of Daylight into Room Without Glare. International Journal of Ambient Energy, 15(2), pp. 73-77, incorporated herein by reference in its entirety. As with Compagnon's design, light enters the louver array through a skyward tilted CPC, and is guided therethrough as shown in FIGS. 4a and 4b, i.e., FIGS. 1 & 2 of Eames et al., which include a graph of ray tracing through the profile. However, Eames and Norton's design has an asymmetric anidolic louver profile, in which the inner half of the louver is composed of a flat section and a circular section, rather than another CPC.
These inner surfaces of the Eames and Norton design may raise several issues that may make this design problematic. For example, the louvers are spaced such that sunlight can pass directly through without being redirected. Also, light is able to reflect off the outer CPC and enter the room at a downward angle. Both of these effects have the potential to cause glare under direct sunlight. The output elevation angle ranges up to 90°, which means that a large portion of the light hits the ceiling immediately adjacent to the facade without travelling very far into the space. The inner flat surface of this louver is diffuse, so light hitting it may be scattered in all directions, further limiting the louver's ability to guide light to the back of the space. Finally, the CPC inlet section rejects light with an elevation angle above 65°, for light normal to the facade in azimuth. As the azimuth angle of the light increases, this cut-off angle drops from 65° down to 0°, for light nearly parallel to the facade. As a result, the system is not able to transmit light from a large portion of the sky. The lower portion of the sky from which the system does successfully pass light is also the first to be blocked by surrounding sky obstructions. Additional attempts were made to incorporate anidolic geometry into a louver array, as shown in FIG. 5, which illustrates additional anidolic louver designs. See FIG. 3 on page 3 of Courret, G., Paule, B. & Scartezzini, J.-L., 1994. Application de l'Optique Anidolique a l'Eclairage Naturel Lateral d'un Nouveau Baitment. In Wärmeschutz Conference. Zurich, 1994, incorporated herein by reference in its entirety.
These designs have some intriguing features but, ultimately, may not be suitable for a deep-plan space. The first two images show the same design at different scales. These louvers collimate light into a very narrow range around horizontal, but since the anidolic curves are not tiled towards the ceiling, approximately half of the light will exit below horizontal. This may be acceptable for a shallow office with a depth 3 or 4 m, but it would likely cause disturbing glare in deeper spaces. Another drawback with this design is that a large fraction of the incoming light is rejected by the steeply inclined plane at the louver's inlet. Finally, the very long and slender shape of the louver may be difficult to produce accurately, especially at the scale contemplated in the middle image. The intent of the rightmost system in the image is to reject high-angle light while admitting low-angle light. This design allows light to exit at a downward angle as well, meaning that all the variants shown in FIG. 5 would likely require an additional shading system if exposed to direct sun.
Referring to FIG. 6, another, more recent, entry into the reflective louver category which does not incorporate a CPC profile is called the LightLouver (U.S. Pat. No. 6,714,352, incorporated herein by reference in its entirety). See also “Information.” LightLouver Daylighting System Homepage. LightLouver LLC. Web. 18 Aug. 2010, incorporated herein by reference in its entirety. This louver is able to collimate the output light closer to horizontal so that light generally penetrates more deeply than with the other systems discussed. However, there are several downsides worth noting. First, the LightLouver system allows low elevation angle sunlight (5° or less) to enter the room at a downward angle. This may or may not be a problem depending on the circumstance of a particular installation site. Second, the entire exterior-facing surface of the louver is a diffusing surface, which rejects a large fraction of the total incoming daylight, leaving less to distribute into the room. This exemplifies a recurring trade off seen in louver systems between the amount of light rejected by the louver and the extent of the emitted light's angular range. Finally, the width-to-height ratio of the LightLouver is rather large at 2.75 (Rogers et al., 2004). More louvers would be required to fill an equivalent window opening than with a design with a smaller aspect ratio.
Static and Dynamic Systems
In general, daylighting systems may be divided into two categories: passive and dynamic. Passive systems are fixed and contain no moving parts. Dynamic systems contain moving parts, which are usually used to track the sun as it moves across the sky.
Since they have no moving parts, passive systems are generally less expensive and require less maintenance than dynamic systems. However, passive systems are typically only effective for a limited range of sun and sky conditions. Moreover, they may at times, allow direct sun to pass through unimpeded, causing glare. As a result, a separate shading system is typically required, which may lead to additional problems resulting from suboptimal control of the shading system.
Dynamic systems are typically used to respond to the dynamic nature of the sun. A common example is the venetian blind, whose slats can be adjusted, manually or automatically, in response to different insolation conditions. When automated, these systems are typically more expensive in both upfront and maintenance costs than their passive counterparts because they require rotating machinery, an accurate control system, and human monitoring. Another limitation is that since most dynamic systems are designed to use the sun's radiation as input, their effectiveness is severely reduced under overcast conditions. In cloudy climates, it may be difficult to justify the additional expense of a sun-tracking dynamic system.